The photodissociation dynamics of HNO3 in the electronic S3 (2 1 A ′ ) state leading to the fragments OH and NO2 was investigated in real time. HNO3 was prepared either in a fluorescence cell at room temperature (LIF probing of OH) or rotationally cold in a molecular beam (probing of NO2 by three-photon ionization). A 2 1 A ′ lifetime of 60–80 fs could be obtained from the experimental results, indicating essentially barrierless dissociation. In addition, secondary dissociation of internally excited nascent fragments NO2  * leading to products NO(X 2 Π) and O(3 P) with a characteristic dissociation time of 2.3 ps was observed. This time is surprisingly long when compared with dissociation lifetimes of NO2 from the literature, obtained after direct photoexcitation. The discrepancy is explained by differences in the preparation conditions of the dissociative state of NO2.

Ultrafast dissociation dynamics in OClO molecules is studied, induced by femtosecond laser pulses in the wavelength region from 386 to 409 nm, i.e., within the wide absorption band to the (approximately)A (2)A(2) electronic state. The decay of the initially excited state due to nonadiabatic coupling to the close lying (2)A(1) and (2)B(2) electronic states proceeds with a time constant increasing from 4.6 ps at 386 nm to 30 ps at 408.5 nm. Dissociation of the OClO molecule occurs after internal conversion within about 250 fs. In addition, a minor channel of direct excitation of the (2)A(1) electronic state has been identified, the lifetime of which increases from a few 100 fs at 386 nm to 2.2 ps at 408.5 nm. Simultaneous excitation of two neighboring vibrational bands in the (approximately)A (2)A(2) state leads to a coherent oscillation of the parent ion signal with the frequency difference of both modes.

Intense (approximate to80 GW cm(-2)) ultrashort (approximate to100 fs) infrared (IR) laser pulses may be employed for excitation of a high frequency (approximate to3500 cm(-1)) local mode vibration in a molecule. Subsequently, an intense (16-256 GW cm(-2)), ultrashort visible (VIS) laser pulse yields electronic excitation with near adiabatic transfer of the vibrational energy, which has been accumulated by the IR pulse. The net result of these sequential IR+VIS laser pulses may be the breaking of a strong molecular bond close to the pre-excited one. In contrast, exclusive excitation by just a visible laser pulse breaks a competing weak bond. The effects of IR+VIS laser pulse control may be considered as an extension of vibrationally mediated chemistry, from ns pulses or continuous wave (cw) excitations to sub-ps laser pulses, and from direct vibrational pre-excitation of the bond to be broken to a neighboring bond, thus exploiting intramolecular vibrational redistribution (IVR) from the pre-excited local mode to the bond to be broken in the electronic excited state. The mechanism is demonstrated by quantum simulations for the model system BaFCH3, where BaF-, FC- and CH3 play the roles of the weak and strong bonds to be broken, and the vibrationally pre-excited CH3 stretch. The theoretical predictions are confirmed experimentally. Various extensions of the control by IR+VIS laser pulses include the control of the branching ratio of weak versus strong bond breaking, as well as isotopomer selectivity depending on the vibrational pre-excitations.

To understand the fundamental photochemical processes in biologically relevant molecular systems, prototype molecules, such as,phenol or indole - the chromophores of the amino acids tyrosine respective trypthophan - embedded in clusters of ammonia or water molecules are an important object of research. Numerous studies have been performed concerning the dynamics of photo-induced processes in phenol-ammonia or phenol-water clusters. As a main result a hydrogen transfer reaction is clearly indicated in phenol(NH 3 ) n clusters, whereas for phenol(H 2 O) n complexes no signature for such a reaction has been found. According to a general theoretical model, a similar behavior is expected for the indole molecule surrounded by ammonia or water clusters. As the primary step an internal conversion from the initially excited ΠΠ * state to a dark Πσ * state is predicted which may be followed by the H-transfer process on the Πσ * potential energy surface. Ab initio, calculations of the indole-water potential energy surfaces are under way now, to elucidate this process in the hetero-cluster and to understand the difference with respect to the indole-ammonia complex.

The ultrafast dynamics of the electronically excited ethylene molecule has been studied in pump–probe experiments with the pump wavelength of 198 nm and the probe wavelength of 790 nm. The lifetime measured for the initially excited ππ∗ state is about 10 fs for C2H4 as well as for C2D4. No secondarily populated states of ethylene can be directly observed despite the high intensity of the probe pulses used. However, the observed time dependence of the fragment ions C2H3+ (C2D3+) and C2H2+ (C2D2+) suggests that they are formed by dissociative ionization via a secondary electronic state of ethylene. This result was confirmed by the time-resolved photoelectron spectra.

The dynamics of the H atom transfer reaction in indole-(NH3)(n), clusters has been studied in femtosecond pump-probe experiments as a function of the pump wavelength (250-282 nm), i.e. for different vibrational excess energies in the excited electronic states. Typical changes of the corresponding time constants in the sub-ps and ps time region were observed for the small clusters sizes (n = 1-3) and compared with those for the larger clusters (n = 4-6). For the heterodimer indole.(NH3)1 the energetic threshold of the reaction has been found at excitation energies of about 4.4 eV. In the sub-ps region we observed no isotope effect for all pump wavelengths used while on the ps time scale the rates for the deuterated clusters d-indole-(ND3)(n) are smaller by a factor up to 4 in comparison to the nondeuterated complexes.

Ab initio calculations on the heterodimer C8H6NH...NH3 are carried out for its ground, the excited pisigma*, and the ground cationic electronic states, enabling the description of hydrogen or proton transfer, respectively. Two-dimensional quantum-dynamical computations on the pisigma* potential surface help one to understand the mechanism and the time scale of the hydrogen transfer. Subsequent decay processes are discussed depending on the vibrational excitation of the ammonium constituent. Finally, the theoretical results obtained are used for the interpretation of the time-dependent signals observed in femtosecond pump-probe experiments.

Applying femtosecond laser pulses at the wavelength of 250 nm the photoinduced processes in indole(H2O)n and d-indole(D2O)n clusters have been studied. An ultrafast (∼50 fs) decay of the initially excited ππ∗ state is tentatively attributed to the internal conversion to the dark πσ∗ state. Subsequent processes are characterized for n=1,2 by a single-exponential signal decay on the ps time scale, while for larger clusters (n⩾3) no significant time dependence is observed. The strong differences with respect to the photophysics of indole(NH3)n clusters are discussed.

The photoinduced H-atom-transfer reaction in indole(NH3)(n) clusters has been analyzed by femtosecond time-resolved photoelectron-photoion coincidence spectroscopy. The different contributions to the measured time-dependent ion and electron signals resulting from ionization by one and two probe photons can be discriminated and analyzed separately. In particular, the distinctively different dynamical behavior observed for clusters with small (n = 1-3) and larger (n greater than or equal to 4) numbers of ammonia molecules is elucidated. For the small clusters an ultrafast process with a time constant of about 150 fs is identified and attributed to internal conversion from the initially excited pipi* state to the pisigma* state. In contrast, for the larger clusters (n greater than or equal to 4) such an initial ultrafast process is not observable probably for Franck-Condon reasons, while a structural rearrangement mechanism after the H transfer on a time scale of 10 ps is clearly recognized.

The femtochemistry of the indole molecule which represents the chromophore of the amino acid trypophan is of particular interest because of its biological relevance. Recent studies (e.g. [1]) of indole in aqueous solution have shown that the photoexcitation leads to a fast charge separation process characterized by the formation of a solvated electron. The microscopic mechanism of the solvation process can be elucidated by gas phase studies of indole-solvent clusters with polar molecules like water or ammonia. Recent ab initio investigations of indolewater clusters have stressed the crucial role of the nonadiabatic coupling between the optically excited ππ* and a low-lying dark πσ* state for the electron transfer process [2]. In first pump-probe experiments on indole (NH3)n clusters with ns laser pulses a H atom transfer reaction has been stated [3].

The intracluster reaction dynamics initiated by femtosecond laser pulses at 263 nm has been studied in nondeuterated and deuterated indole–ammonia clusters. No isotope effect is observed on the sub-ps timescale. On the long-timescale (of few 100 ps), however, the structural reorientation dynamics of the parent as well as the product clusters is prolongated (up to about 3 times) for the deuterated complexes. This effect is interpreted as due to the difference of the torsion modes and due to low energetic barriers between different isomers of both isotopic complexes.

Neutral ammonia clusters (NH3)m are photo-excited to the electronic \(\) state by a deep UV femtosecond laser pump pulse. Within a few hundred femtoseconds a significant fraction of the clusters rearrange to form an H-transfer state (NH3)m-2NH4(3s)NH2 with the subunit NH4 in its 3s electronic ground state. This state is then electronically excited by a time-delayed infrared control pulse of variable wavelength. Finally, a third (probe) pulse in the UV ionizes the clusters for detection. The lifetime of the excited (NH3)m-2NH4(3p)NH2 states is found to vary between 2.7 and 0.13 ps depending on cluster size and excitation energy. It increases drastically upon deuteration. The corresponding cluster size-dependent photoelectron spectra allow us to disentangle the underlying energetics of the excitation and ionization process and reveal additional processes, such as nonresonant ionization or dissociative ionization. The experimental findings suggest that the excited H-transfer ammonia complexes with m > 2 are deactivated by an internal conversion process back to the electronically lowest H-transfer state followed by fast dissociation.

The hydrogen atom transfer reaction in indole (NH3)(n) clusters excited at 263 nm to the S-I(pipi*) state is studied in pump-probe experiments with femtosecond laser pulses. For small clusters the reaction is characterized by two successive processes on distinct time scales: the time constant of the primary process is in the sub-ps region, whereas the secondary decay time growing with the cluster size reflects a relaxation process within 25 to 150 ps. A preliminary model of the H-transfer reaction is discussed.

The H atom transfer reaction in electronically excited indole(NH 3 ) n clusters is studied in pump-probe experiments with femtosecond laser pulses. By applying different probe photon energies we are able to detect the dissociation products (NH 3 ) n - 1 NH 4 for n = 1-6. Furthermore we show that the analysis of the corresponding ion signals is not distorted by contributions from larger cluster ions due to evaporation of NH 3 molecules. The formation times of the products are ca. 140ps for n = 2-4 and about 80ps for n = 5, 6.

Neutral ammonia clusters (NH3)(m) are photo-excited to the electronic (A) over tilde state by a deep UT-V femtosecond laser pump pulse. Within a few hundred femtoseconds a significant fraction of the clusters rearrange to form an H-transfer state (NH3)(m-2)NH4(3s)NH2 with the subunit NH4 in its 3s electronic ground state. This state is then electronically excited by a time-delayed infrared control pulse of variable wavelength. Finally, a third (probe) pulse in the UV ionizes the clusters for detection. The lifetime of the excited (NH3)(m-2)NH4(3p)NH2 states is found to vary between 2.7 and 0.13 ps depending on cluster size and excitation energy. It increases drastically upon deuteration. The corresponding cluster size-dependent photoelectron spectra allow us to disentangle the underlying energetics of the excitation and ionization process and reveal additional processes. such as nonresonant ionization or dissociative ionization. The experimental findings suggest that the excited H-transfer ammonia complexes with m > 2 are deactivated by an internal conversion process back to the electronically lowest H-transfer state followed by fast dissociation.

The energetics and ultrafast dynamics in the H atom transfer configuration of ammonia dimer and trimer clusters have been studied. The clusters are first excited to the electronic à state with a 208 nm femtosecond laser pump pulse. This state is allowed to relax for about 1 ps during which the H-transfer state is formed which is then electronically excited by a time-delayed infrared control pulse at 832 nm and finally ionized with a third femtosecond probe pulse at 416 nm. We have also performed complementary theoretical studies elucidating the experimental findings. For the dimer in the excited NH4(3p)⋯NH2(X̃) state the time-dependent ion signals reveal an isotope-independent short lifetime of about τ6=(130±60) fs which can be explained by a curve crossing with the repulsive NH4(3s)⋯NH2(Ã) state, whereas the trimer signal persists on a time scale being more than one order of magnitude longer and exhibits a very large isotope effect. This is interpreted as being due to internal conversion from the excited state NH3NH4(3p)⋯NH2(X̃) back to the NH3NH4(3s)⋯NH2(X̃) ground state. The analysis of the corresponding photoelectron spectra also confirms the transition energies between the electronic states involved, e.g., ΔE[NH4(3s→3p)⋯NH2]=1.5 eV and ΔE[NH3NH4(3s→3p)⋯NH2]=1.2 eV, as determined by our ab initio calculations.

The intracluster reaction initiated in (BaFCH3)-F-... by excitation to its electronic state near 540 nm proceeds on an extremely short timescale. A time constant of (70 +/- 10) fs is deduced for the rate-determining step in (BaFCH3)-F-... whereas for the deuterated complex (BaFCD3)-F-... this time is enlarged to (100 10) fs. By comparison with the results obtained for the reaction via the excited (A) over bar state at 618 nm a reaction model is suggested which explains the BaF product formation in two successive steps. (C) 2002 Elsevier Science B.V. All rights reserved.